1. Field of the Invention
The present invention relates to an organic EL device, and more particularly, to an improvement of a water-capturing medium placed in the organic EL device in order to maintain stable emitting characteristics which are not affected by water or oxygen for a long period of time.
2. Description of the Prior Art
In general, the organic EL has an emitting part having a laminate structure which is formed by interposing a thin film of organic EL layer containing phosphor organic compound between an anode and a cathode. A hole and an electron are injected into the phosphor organic compound to recombine to generate an exciton. The organic EL device is a spontaneous light-emitting device using emission of light (fluorescence/phosphorescence) when the exciton is inactivated.
It is generally known that a principal problem of the organic EL device is an improvement of the durability of the light emitting part. One of the causes of short lifetime is generation of non-light emitting dot which is called “dark spot.” The non-light emitting dot grows with lapse of time of lighting and the area of the non-light emitting portion enlarges gradually. As a result, the luminance of the light-emitting area decreases. When the diameter of the non-light emitting portion grows up in several tens μm and above, the non-light emitting portion can be visually observed and damages the lifetime of product. It is recognized that a principal cause of the generation of the dark spot is that an organic EL layer constituting the organic EL device reacts with moisture or oxygen in a sealed container to generate and grow the dark spot.
It is, therefore, necessary that the materials constituting the organic EL device do not contain moisture. More particularly, the organic materials used in the emitting part and the sealed container must be processed so as to not contain the moisture. Thus, the organic EL device has been manufactured by a dry process in which the moisture is removed to the utmost from a vacuum container at the time of forming a film or production processes such as a sealing process of the device. However, it is not possible to prevent the dark spot from generating and growing in the EL device at present, because the moisture can not be removed completely.
As described above, the principal problem for the organic EL device is that the moisture inside the container is removed completely to eradicate the dark spot or a small dark spot is not made to grow so as to prolong the life of the EL device. As one of the measures for removing the moisture, a commercially available organic EL device uses an inorganic drying agent as a drying member in a container and seals the container. These techniques were applied for patent and publicly known (for example, Unexamined Patent Publication (Kokai) No. 9-148066.)
FIG. 4 is a side cross sectional view showing the structure of a conventional organic EL device using drying agent as a drying member. The organic EL device 41 has a structure in which a transparent conductive film of ITO(Indium Tin Oxide) is formed on the transparent glass board 42 in a given pattern to form the anode 45.
The organic EL layer 44 of a thin film of organic compound is formed on the anode 45. The cathode 46 of metal thin film of, for example Al—Li, is formed on the upper surface of the organic EL layer 44, and a light emitting part of a laminate comprising the anode 45, organic EL layer 44 and cathode 46 is formed. The organic EL layer 44 is comprised of a hole injection layer containing at least an organic emitting layer, a hole transportation layer, an electron transportation layer, an electron injection layer etc. which are required for EL light emission.
Outer peripheral portion of the glass board 42 is bonded and sealed with the metal sealing cap 43 by means of adhesive 48. The metal sealing cap 43 seals hermetically the anode 45, organic EL layer and cathode 46 on the glass board in an atmosphere of inert gas from which the moisture is removed to the utmost such as dried nitrogen or in a dried atmosphere by dry air so that they are shield hermetically from the air and the moisture.
The recessed area 49 is formed in the inner surface of the metal sealing cap 43 by, for example, press molding. The recessed area 49 contains powdered inorganic drying agent 47 such as barium oxide, calcium oxide, as a drying means. The recessed area 49 containing powdered drying agent 47 is covered by the cap 50. The drying agent placed inside the recessed area 49 adsorbs the moisture remaining in the container to form hydroxide, thereby removing the moisture remaining inside the container or the moisture permeated from the outside.
However, in the case of the conventional organic EL device 41 shown in FIG. 4, the inorganic drying agent 47 contained in the recessed area 49 formed in the metal sealing cap 43 is powdered. Accordingly, if the calcium oxide powder of the drying agent 47 falls on the cathode 46 of the emitting part to adhere to it, aluminum and calcium oxide reacts with water on the cathode, which results in generation of corrosion of thin film cathode 46 made of aluminum. In order to resolve such a problem, the recessed area 49 containing the drying agent 47 must be covered with the water penetrating cap 50 to isolate the drying agent 47 from the emitting part so that the calcium oxide as a drying agent does not adhere to the cathode.
The cap 50 must be placed inside a sealed container. Therefore, the metal sealing cap 43 used has a special shape, thereby complicating the structure. Further, a large space is required for sealing the drying agent and the thickness of the sealing cap 43 must be large. As a result, the entire thickness of the organic EL device becomes large.
Further, since the drying agent 47 is powder, the drying agent 47 is hard to handle so as to prevent contamination by adhesion to the organic EL layer when confining the drying agent 47 in the recessed area 49 of the metal sealing cap 43. That is to say, such conventional powdered drying agent is extremely poor in workability. Further, there has been a problem that air incorporated between powders of the drying agent causes a dark spot to grow due to oxygen in air.
Furthermore, if the generation of the dark spot in the vicinity of the central portion of a display area can be suppressed by a drying member of the sealing cap 43, the generation and growth of the dark spot caused by water permeated from an adhesive layer in the peripheral portion near the sealing part can not be sufficiently suppressed. This is because the sealing part in the peripheral portion of the organic EL device is distant from the drying agent 47 and that it is difficult to select adhesive reliable for different types of materials in the case of bonding the metal cap 43 with the glass board 42.
The inventors have proposed a Japanese patent application which is published by the Unexamined Japanese Patent Publication No. 2002-33187 in order to resolve the conventional problems. The patent application discloses a water capturing medium which can capture the moisture effectively by virtue of chemical reaction without containing oxygen and can suppress the generation and growth of the dark spot in the peripheral portion of organic EL device to form effectively by the use of an organometallic compound highly reactive to the moisture instead of the conventional drying agent.
As shown in FIG. 3, the EL device disclosed in the Unexamined Japanese Patent Publication No. 2002-33187 has a structure of a light emitting area in which the organic emitting layer 34 of an organic EL device is interposed between the anode electrode 35 and the cathode electrode 36 placed opposite to each other on the glass substrate 32. The organic emitting layer has a three-layered structure having the hole injection layer 34a, hole transportation layer 34b and emitting layer-electron transportation layer 34c. The organic emitting layer 34 is placed in a hermetically sealed container formed of the glass substrate 32, sealing cap 33 and sealing part 38. In the hermetically sealed container is placed the water capturing layer 37 to prevent the lamination 34 of the organic EL materials from being contaminated with the moisture. The water capturing layer comprises a film of an organometallic compound represented by the formulae (1) (2) and (3). wherein R is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, M is a trivalent metal atom, and n is an integer of more than one. wherein each of R1, R2, R3, R4 and R5 is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, and M is a trivalent metal atom. wherein each of R1, R2, R3 and R4 is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, and M is a tetravalent metal atom.
The organometallic compound represented by the formulae (1) (2) and (3) is soluble in an organic solvent and liquid at the time of being used. It is applied to the whole inner surface of the sealing cap 33 of flat glass plate to form a water-capturing film 100 μm in thickness used for a water-capturing medium layer. Alternatively, a sealing cap is prepared by forming a recessed area 0.2 to 0.25 mm in depth in the glass board 33 by spot facing, inside of which is formed a water-capturing layer. After an accelerated life test was carried out for the conventional organic EL device at a temperature of 85° C. and humidity of 85%, the growth of dark spot was observed by a microscope. As a result, while the diameter of the dark spot was 1 μm at an initial stage, it was grown slightly up to 10 μm at a central portion after a lapse of 500 hours as shown in FIG. 5. The diameter of the dark spot 10 μm and below can not be visually observed. Accordingly, such an organic EL device had no problem in practical use and was effective.
However, in the case of the sealing cap 33 of flat glass plate of prior art shown in FIG. 3, the solution of the organometallic compounds represented by the formulae (1) (2) and (3) constituting the water-capturing medium layer is diluted by an organic solvent and lipophilic. Since a glass plate is hydrophilic, the water-capturing medium is not compatible with the glass. Accordingly, when the lipophilic water-capturing medium is applied on the hydrophilic glass plate, the water-capturing medium becomes droplets due to surface tension. It was, therefore, not easy to form a thin film pattern. Accordingly, there has been conventionally adopted a method in which the whole of a glass plate is dipped in a solution of water-capturing medium. According to such a method, however, the water-capturing medium is formed in the sealing part of the peripheral portion of the sealing cap. Therefore, such a method has a problem that water permeates from the sealing part after being sealed, and a process for removing the water-capturing medium of the sealing area is required.
On the other hand, there is a proposal that a recessed area is formed in the sealing cap of glass by spot facing processing in order to resolve the above problems. It is, however, necessary to form a recessed part in the portion other than sealing part by a sandblast method or etching method in addition to the space facing. Such a method has problems that long processing time is required and that it not only adversely affects on health of workmen, but also requires management and processing of waste liquor, because the strong acid of hydrofluoric acid is used for etching glass. Further, a part of glass plate is cut or scratched by such a spot facing processing. It is, therefore, necessary to prepare a thicker glass plate in advance, because the thickness of the glass sealing cap decreases and the mechanical strength decreases. Therefore, this method is not advantageous in that the total thickness of the organic EL device becomes large and that costs such as material cost, manufacturing cost rise drastically.